This invention relates to compositions useful for producing glass-ceramic base glasses which are capable of being converted by controlled crystallization into transparent glass-ceramics containing h quartz solid solution crystals and satisfying the following three requirements:
1. The difference between the maximal .DELTA.1/1 value and the minimal .DELTA.1/1 value within the temperature range between -50.degree. C. and +100.degree. C. must be less than or equal to ##EQU1##
2. The linear thermal expansion coefficient within the temperature range 0.degree. C. to +50.degree. C., .alpha..sub.0/50, shall be, in absolute amounts, of a value less than or equal to 0.1.times.10.sup.-6 /K; and the preferred upper limit is 0.065.times.10.sup.-6 /K.
3. The reversible adjustment of the length at 20.degree. C. after different cooling rates from temperatures within the range of admissible applications shall be less than or equal to 0.1.times.10.sup.-5, with a preferred upper limit of 0.14.times.10.sup.-5, when expressed as difference in relative length at 20.degree. C.
Numerous transparent glass-ceramics with low expansion coefficients and containing h quartz solid solution crystals in crystalline phase are known in the relevant literature. Furthermore, for special optical applications, notably for reflecting telescope support materials, glass-ceramics have been developed for which a low thermal expansion in the temperature range from -50.degree. to +100.degree. C. is of highest importance.
Such a material is described for example, in DE-PS No. 14 96 611. Its expansion behavior is characterized by a linear thermal expansion coefficient in the temperature range 0.degree. C. to 300.degree. C., .alpha..sub.0/300, which is in the range -3.ltoreq..alpha..sub.0/300 .ltoreq.3.times.10.sup.-7 /K. In the given examples .alpha..sub.0/300 values which are lower than .vertline.1.0.vertline..times.10.sup.-7 /K are also listed.
DE-PS No. 19 02 432 describes conditions under which easy-to-process glasses may be melted and, by controlled crystallization, converted into the glass ceramic state to produce transparent glass-ceramics having low linear thermal expansion coefficients. According to this disclosure, it is possible to obtain compositions for which the linear thermal expansion coefficients for the temperature range -30.degree. C. to +70.degree. C., .alpha..sub.-30/70, could be adjusted to 0.+-.0.15.times.10.sup.-6 /K by ceramicizing in a wide range from 750.degree. C..ltoreq.T.ltoreq.870.degree. C. and from 4 h.ltoreq.t.ltoreq.100 h. This last condition is of great importance because it guarantees the achievement of one and the same .alpha.-value even for large castings for which homogeneous temperature distribution can be only conditionally obtained even at very slow heating-up rates.
The characterizing criteria for glass-ceramic materials for optical applications which had been applied in the above patent specifications frequently fail to meet modern demands concerning length constancy. Nowadays, instead of specifying one single value for the linear thermal expansion coefficient in a given temperature range, it is necessary to apply the three criteria described at the outset. These characterize the length- and form-constancy.
If the commercial glass-ceramics CERVIT and ZERODUR are tested, it will be found that neither is capable of simultaneously satisfying the aforementioned requirements. The papers by S. F. Jacobs, M. A. Norton, J. W. Berthold III: Thermal Expansion, 1973; AIP Conf. Proc. No. 17, p. 280-293 and J. W. Berthold III, S. F. Jacobs: Appl. Opt. 15 (1976) p. 2344-2347, show that CERVIT fails to meet requirement 1. From the paper by S. J. Bennett: J. Phys. E. Vol. 10 (1977), p. 525-530, it follows that ZERODUR fails to satisfy requirement 3.